In the LTE system, carrier aggregation (CA: Carrier Aggregation) for performing communication by simultaneously using a plurality of carriers is adopted, in which predetermined bandwidths are used as basic units. In carrier aggregation, a carrier which is a basic unit is called a component carrier (CC: component carrier).
When CA is performed, a PCell (Primary cell) that is a reliable cell for ensuring connectivity and an SCell (Secondary cell) that is an appendant cell are set for the user apparatus UE. The user apparatus UE connects to a PCell first, and then, an SCell can be added as necessary. The PCell is a cell similar to an independent cell for supporting RLM (Radio Link Monitoring) and SPS (Semi-Persistent Scheduling) and the like.
Addition and deletion of the SCell is performed by RRC (Radio Resource Control) signaling. Since an SCell is in a deactivated state right after it is set in the user apparatus UE, communication becomes available (scheduling becomes available) only by activating it.
As shown in FIG. 1, for example, in CA of Rel-10 of LTE, a plurality of CCs under the same base station eNB are used. Also, in the CA of Rel.10, as shown in FIG. 2, for example, by aggregating 5 CCs at the maximum, a high data rate is realized by expanding the band.
In Rel-12, Dual connectivity is introduced in which simultaneous communication is performed by using CCs under different base stations eNB to realize high throughputs. In Dual connectivity, the UE performs communication simultaneously using radio resources of physically different base stations eNB. Dual connectivity (to be referred to as DC hereinafter) is a kind of CA, and it is also referred to as Inter eNB CA (inter-base station carrier aggregation), in which Master-eNB (MeNB) and Secondary-eNB (SeNB) are introduced. In CA up to Rel.12, the number of configurable CCs per UE is 5 at the maximum.